Homodyne spin noise spectroscopy and noise spectroscopy of a single quantum dot

Abstract

The steady-state fluctuations of a spin system are closely interlinked with its dynamics in linear response to external perturbations. Spin noise spectroscopy exploits this link to extract parameters characterizing the dynamics without needing an intricate spin polarization scheme. In samples with an accessible optical resonance, the spin fluctuations are imprinted onto a transmitted linearly polarized quasi-resonant probe laser beam according to the optical selection rules, making an all-optical observation of spin dynamics possible. The beam’s detuning and intensity determine whether the system is probed at thermal equilibrium or under optical driving. The technique is uniquely applicable for studying single quantum dots, where a charge carrier’s spin and occupancy dynamics can be observed simultaneously.

This thesis presents a step-by-step derivation of the shape and statistical properties of experimental spectra and highlights the experimental limitations faced by the technique at very low probe intensities through uncorrelated broadband technical noise contributions. Optical homodyne amplification is evaluated in a proof-of-principle experiment to determine whether this limitation can be overcome at low frequencies < 5 MHz. Unlike previous attempts, the presented proof-of-principle experiment demonstrates that shot-noise limited spin noise measurements are possible in low-frequency ranges down to ≳ 100 kHz. For even lower frequencies, the suppression of laser intensity noise by the limited common-mode rejection of conventional balanced detectors is found to be the limiting contribution.

In the second part of the thesis, optical spin noise spectroscopy is used to conduct a long-term study of spin and occupancy dynamics of an individual hole spin confined in an (In,Ga)As quantum dot with high radial symmetry in the high magnetic fields regime. For magnetic fields ≳ 250 mT, the splitting of the Zeeman branches with an effective g-factor of 2.159(2) exceeds the quantum dot’s trion resonance’s homogeneous line width of 6.3(2) μeV, revealing a rich spectral structure of spin and occupancy dynamics. This structure reveals a so far neglected contribution of an internal photoeffect to the charge dynamics between the quantum dot and its environment. Previously developed theoretical modeling is extended to incorporate the photoeffect and successfully achieves excellent qualitative correspondence with experimental spectra for almost all detuning ranges. The photoeffect shuffles the charge from and into the quantum dot with two distinct rates. Within the model, the previously required Auger process is unnecessary to describe the experimental data. The rates of discharging and recharging the quantum dot are determined to be on the order of 12(7) kHz·μm²·nW⁻¹ and 6(2) kHz·μm²·nW⁻¹, respectively.

For magnetic fields < 500 mT, very long T1 hole spin relaxation times ≫ 1 ms are observed, while above 500 mT, T1 falls to 5(2) μs at 2.5 T, qualitatively confirming the theoretical prediction of a single-phonon mediated relaxation process. Furthermore, the electron spin relaxation time T1 in the trion state shows no pronounced dependence on magnetic fields above 500 mT and stays at a constant value of 101(2) ns. The saturation intensity of the transition also does not depend on the magnetic field and stays at a constant value of 4.8(7) nW·μm⁻².